We describe the synthesis of ZnS-capped CdSe semiconductor nanocrystals using organometallic reagents by a two-step single-flask method. X-ray photoelectron spectroscopy, transmission electron microscopy and optical absorption are consistent with nanocrystals containing a core of nearly monodisperse CdSe of 27-30 Angstrom diameter with a ZnS capping 6 +/- 3 Angstrom thick. The ZnS capping with a higher bandgap than CdSe passivates the core crystallite removing the surface traps. The nanocrystals exhibit strong and stable band-edge luminescence with a 50% quantum yield at room temperature.